Optical circuit fabrication method and device

ABSTRACT

A photonic light circuit device is described that comprises a semiconductor substrate and two or more optical components wherein one or more hollow core optical waveguides are formed in the semiconductor substrate to optically link said two or more optical components. The PLC may comprise a lid portion and a base portion. The PLC can be adapted to receive optical components or optical components may be formed monolithically therein. Coating with a reflective layer is also described.

The present invention relates generally to integrated optics devices,and more particularly to improved photonic light circuit (PLC) devices.

Photonic circuit modules form an integral part of many opticalcommunication, sensor and instrumentation devices. In such photoniccircuit devices a number of optical components are rigidly held in placeand waveguides, typically lengths of optical fibre, are used tooptically connect the components as required. The optical components andinterconnecting fibres are held in place on a suitable substrate.

Silicon optical benches (SiOBs) are one example of an assemblytechnology for photonic circuits. As the name suggests, SiOBs areoptical benches formed from silicon or a similar semiconductor material.Grooves and slots are etched in the silicon material, usingmicro-fabrication processes, to hold the various optical components. Thehigh accuracy of the micro-fabrication process allows the opticalcomponents and optical fibres to be precisely aligned relative to oneanother in the various slots and grooves. This provides so called“passive alignment” of the components and reduces the need to activelyensure the various components of the optical circuit are aligned withone another. Light may also be directed between the various opticalcomponents using free space optics such as lenses etc.

It is also known, for example see U.S. Pat. No. 4,902,086 and EP0856755,that it is possible to deposit various layers of material to formwaveguides that are integral with the SiOB. Typically a base layer, suchas silica, is formed on the silicon substrate. A layer of doped silicawith a high refractive index, i.e. the core layer, is then deposited ontop of the low refractive index base layer. The core layer is patternedto form appropriate waveguides. Optionally, an upper cladding layer oflow refractive index material is also deposited on the patterned corelayer. In other words, waveguides are formed directly on the siliconsubstrate rather than being fabricated as separate optical fibres.

A disadvantage of known photonic circuit devices, including those basedon SiOBs, is the high degree of accuracy with which each opticalcomponent has to be aligned with the associated waveguides to ensure anefficient optical connection. In addition to ensuring accurate physicalalignment of the optical fibres and optical components, it is alsonecessary to minimise unwanted reflections from the end of each silicawaveguide. This requires refractive index matching of the waveguides tothe optical components, or the use of a gel or antireflection coating.Lenses may also be required to facilitate the free space coupling oflight between components. These requirements increase the complexity,and hence cost, of photonic circuit fabrication.

It is an object of the present invention to mitigate at least some ofthe disadvantages described above.

According to a first aspect of the invention, a photonic light circuitdevice comprises a semiconductor substrate and two or more opticalcomponents wherein one or more hollow core optical waveguides are formedin the semiconductor substrate to optically link said two or moreoptical components.

The present invention is advantageous over prior art photonic circuitdevices as it removes the requirement to provide optical fibres forinterconnects between components or to deposit layers of material toform solid core waveguides. This invention provides a photonic lightcircuit (PLC) that is easier to fabricate, and hence lower cost, thanprior art devices.

A further advantage of linking the components with hollow opticalwaveguides is the increased optical power the circuit can handle overprior art photonic circuits that use solid core (typically Silica orsilicon) waveguides to interconnect the optical components. Furthermore,index matching gels or epoxies, or antireflection coatings are notrequired on the faces of the waveguides.

The hollow waveguides are formed so as to guide light between opticalcomponents of the PLC. The optical components are any devices that willcreate, detect or act on an optical signal; for example beamsplitters/recombiners, etalon structures, lenses, waveplates,modulators, lasers, photo-detectors, or actuated optical components. Theterm optical component should also be taken to include opticalstructures, such as surface grating profiles etc, that are formed in orfrom the hollow waveguides. The hollow core waveguides may be planar ortwo dimensionally guiding as described below. An optical component mayalso be an optical fibre cable; for example an optical fibre cable thatis used to couple light in to, or out from, the PLC.

Semiconductor substrates can be etched to a high degree of accuracyusing micro-fabrication techniques. The substrate may advantageouslycomprise a multiple layer wafer; for example SiGe orsilicon-on-insulator (SOI) or silicon-on-glass. A person skilled in theart would recognise that micro-fabrication techniques typically involvea lithography step to define a pattern, followed by an etch step totransform the pattern in to one or more layers on, or in, the substratematerial. The lithography step may comprise photolithography, x-ray ore-beam lithography. The etch step may be performed using ion beammilling, a chemical etch, a dry plasma etch or a deep dry etch (alsotermed deep silicon etch). Micro-fabrication techniques of this type arealso compatible with various layer deposition techniques such assputtering, CVD and electro-plating.

Advantageously, the semiconductor substrate comprises one or morealignment slots, each alignment slot being adapted to receive inalignment an optical component. The alignment slots are formed to theshape required to accept the optical components and may thus bedeeper/shallower and/or wider/narrower than the hollow core opticalwaveguides.

The alignment slots can thus be fabricated with sufficient accuracy toalign the optical component they receive. Placing an optical componentin such an alignment slot inherently aligns the optical component and acomponent alignment or adjustment step is not required. Conventionalpick and place techniques of the type used in the manufacture ofelectronic circuits and the like could be used to place the opticalcomponents in the alignment slots.

Alternatively, pick and place techniques may provide the necessaryalignment. For example, a component could be accurately aligned whenplaced and then fixed (e.g. glued) to remain in alignment.

The alignment slots and (especially) the optical components aremanufactured with a certain size tolerance. The coupling efficiencybetween a optical component and an associated hollow core opticalwaveguide will reduce as the angular error of alignment of the opticalcomponent with respect to the hollow core waveguide increases. However,reduction of the cross-sectional dimensions of the hollow core waveguidewill increase the acceptable angular alignment tolerance, albeit at theexpense of slightly increased losses in the optical waveguide due to thereduced core dimensions and increased (tighter) lateral alignmenttolerances. Therefore, knowledge of the alignment tolerances that willbe achieved with a certain optical component (e.g. from knowledge of themanufacturing tolerances of the optical component) will permit thedimensions of the hollow core waveguide to be selected to ensure a highcoupling efficiency.

The alignment slots may also be formed so as to clamp a solid coreoptical fibre in place thereby allowing optical inputs/outputs to bemade to the PLC. Stepped optical fibre alignment slots may also beprovided to hold both the buffer layer and the cladding. The alignmentof the core of a hollow core optical fibre with a hollow core waveguideon the PLC, achieved for example by clamping the optical fibre claddingin a alignment slot, would be especially advantageous as the air core toair core connection would be free from any unwanted reflections.

To provide efficient coupling between the core of an optical fibre and ahollow core waveguide of the PLC, the cross-section of the hollow corewaveguide should be appropriate for the cross-section of the opticalfibre core. In the case of solid core fibres, leakage into the claddingmeans that the width of the mode carried by the fibre is actuallygreater than the core diameter; for example typically the 10 μm solidcore of a single mode glass fibre has a total field width of around 14μm diameter. If the mode width is different to that of the hollow corewaveguide, lenses (e.g. ball or GRIN rod etc) can be used to expand orreduce the optical field to enable light to be coupled to/from fibreswith a different size core to that of the hollow core waveguide of thePLC. Fibre ends of solid core fibres may be anti-reflection.

Conveniently, one or more of the two or more optical components areformed from the material of the semiconductor substrate; i.e. monolithiccomponents may be formed.

Alternatively, some or all of the optical components that make up thePLC, and which are interconnected via the hollow core waveguides formedin the semiconductor substrate, may be attached to the semiconductorsubstrate as described above; in other words, a hybrid device may beformed.

At least one of said two or more optical components may advantageouslycomprise a micro-electro-mechanical (MEMS) device. The MEMS componentmay be hybrid or monolithic. MEMS is taken to include micro-machinedelements, micro-systems technology, micro-robotics andmicro-engineering. Examples of MEMS optical components include alignmentelements, pop-down Fresnel lenses, gyroscopes, moveable mirrors,tuneable Fabry-Perot cavities, adaptive optics elements, switches,variable optical attenuators, filters etc.

Conveniently, the semiconductor substrate forms a base portion of thephotonic light circuit device and a lid portion is additionally providedin order to form said hollow core optical waveguides.

Advantageously, one or more optical components are attached to the lidportion. Optical components may be mounted on the lid alone, on the baseportion alone, or on both the lid and the base.

The lid portion may be formed from semiconductor material, such assilicon, and advantageously one or more optical components may be formedthereon. Alternatively, the lid portion may be formed from glass.Preferably, the lid should have the same thermal expansion properties asthe substrate; for example, by the lid being formed from the samesemiconductor material as the substrate.

In the case of lid mounted components, the base portion is etched toform the hollow waveguide structures and to provide recesses for opticalcomponents that are formed from, or attached to, the lid portion.Mounting the lid portion on the base portion allows the opticalcomponents to be brought into alignment with the optical waveguides ofthe base portion. A person skilled in the art would recognise thatvarious techniques, such as precision alignment mating parts or wafer orchip alignment tools, may be provided to ensure accurate alignment ofthe lid and base. Alternatively, some or all of the optical componentsmay be directly mounted in alignment slots formed in the base portion.This enables the lid portion to be mounted on the base portion without arequirement to precisely align the lid and base portions.

Conveniently, the lid portion carries a reflective coating. Thereflective coating may cover all, or just selected parts, of the lidportion as required. Advantageously, the reflective coating maycomprises a layer of material having a refractive index lower than thatof the waveguide core within the operating wavelength band; for example,gold, silver or copper. Alternatively, one or more layers of dielectricmaterial or a layer of Silicon Carbide may be provided.

A person skilled in the art would recognise how the lid portion and baseportion cold be bonded together. For example, an intermediate layer suchas conductive or non-conductive epoxy could be used. Alternatively, andin the case of a metal layer being used as a low refractive index layer,a metal-semiconductor eutectic bond could be formed. Glass frittechniques could be employed to bond the lid to the semiconductor baseportion or, if the lid portion is formed from glass, anodic techniquescould be used.

Advantageously, the semiconductor substrate comprises silicon. This maybe provided in a variety of forms, for example in wafer form (e.g. Si,silicon-on-insulator or silicon-on-glass) or as a epitaxial layer (e.g.SiGe or GaAs) on a Si substrate. Advantageously, SOI is used.

Conveniently, the optical properties of a first internal surface formingone or more of the hollow core optical waveguides are different to theoptical properties of a second internal surface forming that hollow coreoptical waveguide. This enables hollow waveguides to be formed that moreefficiently guide light of a certain polarisation as described in moredetail with reference to FIG. 6 below.

Advantageously, at least some of the internal surfaces of said one ormore hollow core optical waveguides carry a reflective coating.

The reflective coating may advantageously comprise a layer of materialhaving a refractive index lower than that of the waveguide core withinthe operating wavelength band.

The layer of material having a refractive index lower than the hollowwaveguide core provides total internal reflection (TIR) of light withinthe PLC waveguides, thereby decreasing the amount of optical loss.

It should be noted that when hollow core optical waveguide structuresare produced, the hollow core is likely to fill with air. Herein therefractive index of the core is thus assumed to be that of air atatmospheric pressure and temperature (i.e. n≈1). However, this should beseen in no way as limiting the scope of this invention. The hollow coremay contain any fluid (for example a liquid or an inert gas such asnitrogen) or be a vacuum. The term hollow core simply means a core whichis absent any solid material. Also, the term total internal reflection(TIR) shall be taken herein to include attenuated total internalreflection (ATIR).

Conveniently, the reflective material carried on the internal surface ofthe hollow core optical waveguides is a metal such as gold, silver orcopper. Metals will exhibit a suitably low refractive index over awavelength range that is governed by the physical properties of themetal; standard text books such as “the handbook of optical constants”by E. D. Palik, Academic Press, London, 1998, provide accurate data onthe wavelength dependent refractive indices of various materials. Inparticular, gold has a refractive index less than that of air atwavelengths within the range of around 500 nm to 2.2 μm; thisencompasses wavelengths within the important telecommunications band of1400 nm to 1600 nm. Copper exhibits a refractive index less than unityover the wavelength range of 560 nm to 2200 nm, whilst silver hassimilar refractive index properties over a wavelength range of 320 nm to2480 nm.

A layer of metal may be deposited using a variety of techniques known tothose skilled in the art. These techniques include sputtering,evaporation, chemical vapour deposition (CVD) and (electro orelectro-less) plating. CVD and plating techniques allow the metal layersto be deposited without significant direction dependent thicknessvariations. Sputtering using a rotating sample and/or source would alsoprovide even coverage. Plating techniques are especially advantageous asthey permit batch (i.e. multi-substrate parallel) processing to beundertaken.

A skilled person would recognise that adhesion layers and/or barrierdiffusion layers could be deposited on the hollow waveguide prior todepositing the layer of metal. For example, a layer of chrome ortitanium could be provided as an adhesion layer prior to the depositionof gold. A diffusion barrier layer, such as platinum, may also bedeposited on the adhesion layer prior to gold deposition. Alternatively,a combined adhesion and diffusion layer (such as titanium nitride,titanium tungsten alloy or an insulating layer) could be used.

Conveniently, the reflective coating may be provided on the internalsurfaces of the hollow waveguides (including any lid portion) by anall-dielectric, or a metal-dielectric, stack. A person skilled in theart would recognise that the optical thickness of the dielectriclayer(s) provides an interference effect that will determine thereflective properties of the coating. The dielectric material may bedeposited by CVD or sputtering or reactive sputtering. Alternatively, adielectric layer could be formed by chemical reaction with a depositedmetal layer. For example, a layer of silver could be chemically reactedwith a halide to produce a thin surface layer of silver halide.

In other words the reflective coating may. be provided by anall-dielectric, or a metal-dielectric, stack. A person skilled in theart would recognise that the optical thickness of the dielectriclayer(s) gives the required interference effects and thus determines thereflective properties of the coating. The reflective properties of thecoating may also be dependent, to some extent, on the properties of thematerial in which the hollow core waveguides are formed. Hence, theunderlying semiconductor substrate may also form a base layer, and be apart of, any such multiple layer dielectric stack.

Furthermore, the layer of material carried on the internal surface ofthe hollow core waveguides is conveniently Silicon Carbide.

As described above, the additional layer of low refractive indexmaterial can be selected to provide efficient operation at any requiredwavelength. Silcon Carbide has a refractive index of 0.06 at 10.6 μm,making such material particularly suited for inclusion in devicesoperating at such a wavelength.

Advantageously, at least one of the one or more hollow core opticalwaveguides have a substantially rectangular (which herein shall includesquare) cross-section. A square, or almost square, cross-section hollowcore waveguide provides a waveguide in which the losses aresubstantially polarisation independent and is preferred when thepolarisation state of the light is unknown or varying.

Preferably, the rectangular hollow core optical waveguide has a firstcross-sectional dimension parallel to a first waveguide wall and asecond cross-sectional dimension orthogonal to said firstcross-sectional dimension wherein the first cross-section dimension isat least 5% or 10% or 15% or 25% or 50% greater than the secondcross-sectional dimension. As described with reference to FIG. 7 dbelow, such a waveguide is preferred for linearly polarised light ofknown polarisation.

Advantageously, the refractive indices of the surfaces defining the atleast one rectangular internal cross-section hollow core opticalwaveguide are substantially equal. This can reduce polarisationdependent losses in the waveguide.

Preferably, opposite surfaces forming the rectangular internalcross-section hollow core optical waveguide have substantially equaleffective refractive indices and adjacent surfaces forming therectangular internal cross-section hollow core optical waveguide havedifferent effective refractive indices. As described with reference toFIGS. 7 a to 7 c below, tailoring the refractive indices of opposingpairs of waveguide walls enables transmission losses to be reduced whenguiding light of a known linear polarisation.

Advantageously, a pair of opposed surfaces of the rectangular internalcross-section hollow core optical waveguide carry a high refractiveindex coating. This provides the high refractive index preferred whens-polarised light is to be reflected as described below.

The semiconductor material of the substrate may also be doped to modifyits optical properties to reduce hollow core waveguide losses.

Conveniently, at least one of the one or more hollow core opticalwaveguides support fundamental mode propagation. Also, at least one ofthe one or more hollow core optical waveguides may advantageouslysupport multi-mode propagation. Preferably, the multi-mode region is ofa length such that re-imaging occurs as described in more detail below.

A person skilled in the art would recognise that the shape anddimensions of the hollow waveguide will affect the associated opticalguiding properties. For example, tapered hollow waveguides could be usedto provide a beam expansion or compression function. The high resolutionwith which hollow core waveguides can be fabricated usingmicro-fabrication techniques allows the guiding properties to betailored as required to optimise PLC operation. A person skilled in theart would however recognise that the shape of the hollow core opticalwaveguides may be dictated to some extent by the type ofmicro-fabrication process used. For example, v-grooves can readily bewet etched in [100] silicon whilst rectangular waveguides can be easilyprovided in [110] silicon by wet etching. However, deep reactive ionetching (DRIE) provides the greatest ease of manufacture.

Advantageously, the device is provided for operation with radiationwithin the wavelength ranges of 0.1 μm to 20 μm, 0.8 μm to 1.6 μm ormore preferably in the range of 1.4 μm to 1.6 μm. The optical propertiesof gold, silver and copper coating therefore make these metalsparticularly suited to inclusion in PLC devices for operation in thetelecommunications wavelength band (i.e. for use with wavelengthscentred around 1.55 μm). Advantageously, the device may operate in thethermal infra-red bands of 3-5 μm or 8-12 μm.

Conveniently, the semiconductor substrate comprises at least onealignment slot arranged to receive an optical fibre cable and tooptically couple said optical fibre cable with one of said one or morehollow core optical waveguide of the semiconductor substrate.

Furthermore, a mode matching means may be advantageously provided in thevicinity of the alignment slot to allow coupling between the modes of anoptical fibre and the analogous modes of a hollow core optical waveguideof a different core diameter. For example, in the case of a fundamentalmode optical fibre the mode matching means couples the fundamental modeof the fibre and the fundamental mode of the hollow core waveguide. Inthe case of multi-mode propagation, the mode spectrum of the opticalfibre is matched to the mode spectrum of the hollow core waveguide. Themode matching means may advantageously comprise a GRIN rod, a ball lens,a conventional lens or a Fresnel lens.

Alternatively, the alignment slot may be arranged to received a lensedoptical fibre.

Preferably, the alignment slot is arranged to receive a hollow coreoptical fibre. The optical fibre may be multi-mode or single mode.

Advantageously, at least one of said two or more optical componentscomprises a mirrored surface that is angled to direct light out of theplane of the semiconductor substrate. The mirrored surface may be amonolithic (e.g. an angled semiconductor surface as described in FIG.15) or hybrid arrangement. In other words, the PLC is not restricted toguiding light in the plane of the substrate surface. Light may bedirected out of the plane of the substrate. For example, stacked orthree dimensional PLCs could be fabricated in accordance with thisinvention.

Conveniently, the PLC may further comprise at least one micro-wavecomponent and/or a hollow core microwave waveguide. In other words, anoptical/microwave hybrid circuit may be provided.

According to a second aspect of the invention, a base portion for aphotonic light circuit comprises a semiconductor substrate having one ormore hollow channels formed therein, wherein said base portion isarranged such that when combined with an appropriate lid portion atleast one hollow core optical waveguide is formed.

Conveniently, at least one slot is formed in the semiconductor substrateof the base portion to receive in alignment an optical component.

According to a third aspect of the invention, a base portion for aphotonic light circuit comprises a semiconductor substrate in which oneor more hollow waveguide channels and at least one slot to receive inalignment an optical component are formed.

According to a fourth aspect of the invention, a method of fabricating aphotonic light circuit comprising the steps of taking a base portionaccording to the second or third aspects of the invention and attachinga lid thereto.

According to a fifth aspect of the invention a method of fabricating aphotonic light circuit device comprises the step of micro-fabricatingone or more hollow channels in a semiconductor substrate that aresuitable, in use, for acting as hollow core waveguides.

Conveniently, the additional step of fabricating slots in thesemiconductor substrate for the appropriate passive alignment of opticalcomponents therein is performed. The slots may be fabricated usingmicro-fabrication techniques, or by precision engineering techniquessuch as laser machining.

Advantageously, the method comprises the additional step of coating theinternal surfaces of the hollow channel(s) with a layer of materialhaving a refractive index lower than that of the waveguide core withinthe operating wavelength band.

According to a sixth aspect of the invention, a method of forming aphotonic light circuit comprising the steps of (a) taking asemiconductor substrate in which at least one hollow core opticalwaveguide and at least one slot to receive an optical component areformed, and (b) introducing an optical component into the at least oneslot, whereby the step of introducing the optical component into the atleast one slot also acts so as to align said optical component.

According to a seventh aspect of the invention, a master suitable forforming a pattern in a layer of deformable material is provided whereinthe master comprises semiconductor material appropriately patterned toform in said deformable material at least one hollow waveguide channeland at least one alignment slot wherein said at least one alignment slotis arranged to receive in alignment an optical component.

Alternatively, a master could be formed in semiconductor material thatallows production of a sub-master. The sub-master may then be used toform the required pattern in a deformable material to define a PLC. Amaster or sub-master may also be used as a mould to form the requiredpattern in a fixable layer.

According to an eighth aspect of the invention a method of forming aphotonic light circuit comprising the steps of; (a) using a masteraccording to the seventh aspect of the invention to permanently form apattern in a layer of deformable material and (b) introducing at leastone optical component into the, at least one alignment slot formed inthe deformable material.

A photonic light circuit device is thus described that comprises asemiconductor substrate wherein one or more hollow core opticalwaveguides are formed in the semiconductor substrate.

The invention will now be described, by way of example only, withreference to the accompanying drawings in which;

FIG. 1 shows a typical prior art SiOB comprising a plurality of opticalcomponents;

FIG. 2 shows a integrated solid core waveguide as used in certain priorart SiOB devices;

FIG. 3 shows a portion of a PLC according to the present invention;

FIG. 4 shows a number of hollow core waveguides according to the presentinvention;

FIG. 5 gives cross-sectional views of various hollow core waveguides;

FIG. 6 shows the Fresnel reflectance coefficient of a copper coatedsurface for s-polarised and p-polarised light;

FIG. 7 provides a cross sectional view of four additional hollow corewaveguides;

FIG. 8 a shows a hollow core beamsplitter, FIG. 8 b shows a Brewsterplate and FIG. 8 c shows the reflectivity of silicon as a function ofthe angle of incidence of s-polarised and p-polarised light;

FIG. 9 illustrates a monolithic lens (FIG. 9 a) and focussing mirror(FIG. 9 b) fabricated in a Silicon substrate;

FIG. 10 illustrates a PLC in which light is coupled into and out ofoptical fibre cables;

FIG. 11 shows a tapered waveguide formed in a silicon substrate;

FIG. 12 shows a hollow core wavelength de-multiplexer formed in asilicon substrate;

FIG. 13 shows a hollow core proximity coupler;

FIG. 14 show a PLC having both hollow core and solid core waveguides;

FIG. 15 shows a PLC having a mirrored surface angled to couple light outof the plane of the substrate;

FIG. 16 show the optical loss of hollow core waveguides uses in PLCs ofthe present invention;

FIG. 17 illustrates the effect of angular misalignment;

FIG. 18 illustrates the effect of lateral misalignment; and

FIG. 19 shows a means of holding a component in alignment in analignment slot.

Referring to FIG. 1, typical prior art silicon optical bench apparatusis shown.

FIG. 1 a shows a silicon optical bench 2 having a micro-fabricatedhollow channel 4 and a pair of solder connectors 6. The silicon opticalbench 2 is configured to hold a laser 8 and a silica optical fibre cable10.

FIG. 1 b shows the silica optical fibre 10 and the laser diode 8 mountedon the silicon optical bench 2. The hollow channel 4 is formed with highenough precision so that the optical output from the laser 8 isprecisely aligned with the end of the silica optical fibre 10. Thesolder connectors 6 provide an electrical connection and attach thelaser diode 8 to the substrate.

To minimise unwanted reflections from the end of each silica waveguidean antireflection coating (not shown) is provided. Alternatively, thesilica waveguides can be refractive index matched (e.g. using an indexmatching gel) and connected directly to each of the optical components.The requirement for anti-reflection coatings and/or index matching addsto the cost of the overall device, and makes fabrication more complexand time consuming.

Although, for simplicity, a single optical fibre cable (i.e. silicaoptical fibre cable 10) and a optical component (i.e. the laser 8) areshown in FIG. 1, a person skilled in the art would recognise thatcomplex multi-component photonic circuits can be fabricated using thesame principle. Many optical components can be located on the siliconoptical bench, and optical links can be established between thecomponents using various lengths of silica optical fibre waveguides. Theoptical components may include, for example, optical modulators, beamsplitters, beam recombiners, detectors etc.

Referring to FIG. 2, a prior art integrated optical waveguide for use aspart of a silicon optical bench is shown.

A low refractive index silica layer 20 is deposited on the siliconoptical bench substrate 22. A high refractive index layer of dopedsilica is formed on the silica layer 20, and a high refractive indexwaveguide core 24 is formed by etching away portions of the high indexlayer of doped silica. A capping layer 26 of low refractive index silicacovers the high refractive index waveguide core 24.

The high refractive index waveguide core 24 acts as an opticalwaveguide, and the high refractive index of the core compared to thecladding provides light guiding by total internal reflection. Thisprovides a optical waveguide that is integral with, and not merely heldin connection with, the silicon optical bench. Solid core integraloptical waveguides are thus a known alternative to optical fibresmounted in grooves on a silicon optical bench. However the use ofintegrated optical waveguide does not lessen the requirement to indexmatch the waveguides to the optical components, or provideanti-reflection coatings. Depositing additional layers of material onthe silicon substrate also increases the fabrication complexity of thephotonic circuit.

Referring to FIG. 3, a hollow core waveguide photonic light circuit(PLC) 40 that forms part of a device of the present invention is shown;FIG. 3 a giving a perspective view of the PLC and FIG. 3 b showing across-section of the PLC along the dashed line marked “A” in FIG. 3 a.

The hollow core waveguide PLC 40 comprises a silicon base 42 and asilicon lid 44. A laser 8 is attached to and aligned in the silicon base42. Light emitted by the laser 8 is coupled in to the single mode hollowcore waveguide 46 that is formed by the silicon base and the silicon lid44. In other words, hollow core waveguides are formed directly in thesilicon from which the PLC base and lid are fabricated. For simplicity,electrical connections to the laser 8 are not shown as a person skilledin the art would recognise the various ways in electrical connectionscould be made; for example, track implantation using diode isolation inthe base 42.

The hollow core waveguide 46 of FIG. 3 can be seen to have a rectangularcross section. The use of rectangular waveguides (herein the termrectangular shall include square) having a substantially equal depth andwidth reduces polarisation dependent losses which can provideadvantageous in many telecommunication applications.

Although rectangular waveguides are shown, the waveguide cross-sectioncould be shaped as required. For example, circular or paraboliccross-section or V-shaped waveguides could be formed in the silicon baseusing appropriate etching techniques. Hollow waveguide structures couldalso be formed in the silicon lid 44. However, this requires both thebase and the lid to be patterned and also means the lid and base have tobe precisely aligned. The dimensions of the hollow core waveguide can beselected to support fundamental mode or multi-mode propagation asrequired and are described in more detail below.

In the example described with reference to FIG. 3, Silicon is used toform the PLC as it can be etched to a very high degree of accuracy usingmicro-fabrication techniques of the type known to those skilled in theart. However, a person skilled in the art would also recognise that anymicro-fabricated semiconductor material could be employed to form a PLCof the present invention.

The laser 8 is a separate component that is bonded to the silicon base42; in other words it is a hybrid arrangement. A person skilled in theart would also recognise that it would be possible to bond the laser 8to a lid, or to fabricate optical components in the silicon itself.Although only a laser 8 is described with reference to FIG. 3, manyoptical components could be located or formed and/or aligned on thesilicon base or lid. Alignment slots formed in the lid may also be usedto receive in alignment optical components. This technique thus allowscomplex multi-component PLCs to be fabricated. The optical componentsmay include, for example, optical modulators, beam splitters, beamrecombiners, detectors, gratings, mirrors, GRIN (graded refractiveindex) lenses etc. Examples of some of the types of optical componentsthat could be formed in a PLC of the present invention are described inmore detailed below.

To maximise optical transmission through the hollow core waveguide 46, alayer of gold 48 is provided on the internal surface of the hollow corewaveguides 46. The deposition of a layer of gold onto the silicon baseand lid can be readily achieved, for example using appropriate metaldeposition techniques such as sputtering or plating.

The lid may be bonded to the base in a variety of ways known to thoseskilled in the art. Areas of silicon that do not form part of the hollowoptical waveguides may be left exposed on the lid portion and/or thebase portion, and the lid and base may be bonded via a gold-siliconeutectic bond. Silver loaded epoxy, solder or polymer adhesive may alsobe used to bond the lid and base. The lid may only cover a part of thebase as required.

The presence of the layer of gold 48 provides ATIR within the hollowcore device for light with a wavelength within the telecommunicationswavelength band (i.e. for wavelengths around 1.55 μm). At thesetelecommunication wavelengths, gold has the required refractive indexproperties of n<1.

Although a gold layer 48 is described, a person skilled in the art wouldrecognise that any material having a refractive index less than air (orwhatever is contained within the cavity) at the wavelengths at which thewaveguide is to be operated could be deposited on the surfaces definingthe hollow core waveguide. The refractive indices of different materialscan be found in various publications, such as “the handbook of opticalconstants” by E. D. Palik, Academic Press, London, 1998. Metalstypically posses a refractive index less than air over a givenwavelength range; the particular wavelength range depending on thephysical properties of the metal.

It should be noted that although the layer of gold 48 provides ATIR,coating the hollow core waveguide 46 with an additional layer of lowrefractive index material is not essential. The refractive index ofsilicon is around 3.4 at wavelengths between 0.5 μm and 300 μm, andhence hollow core (i.e. air filled) waveguides formed from uncoatedsilicon will not provide ATIR of light within such a wavelength range.However, uncoated silicon will still provide light guiding by way ofFresnel reflection. Hollow core waveguides that use Fresnel reflectionsto guide light will introduce more optical loss than waveguides thatprovide TIR, but in certain situations this increased level of opticalloss is acceptable.

If a reflective coating is provided, the substrate may be formed from amicro-fabricated material other than a semiconductor. For example,plastic waveguide devices may be fabricated by techniques including hotembossing or injection moulding. The technique involves forming amaster. The master may be formed in semiconductor material, such assilicon, using a deep dry etch. Alternatively, the master may be formedby electro deposition of layers using the LIGA or UV LIGA technique.Once the master is formed, the hollow core waveguides may be formed in aplastic substrate by stamping (i.e. pressing) or hot stamping. A mastermay also be fabricated which is suitable for forming a sub-master thatcan be used to form the hollow core waveguides in the plastic substrate.Hollow plastic waveguides can thus be formed and coated with areflective coating. The plastic hollow core waveguides that carry thereflective coating may also be formed from plastic or a polymer. Forexample, the hollow core waveguides could be formed using a lithographicprocess on a “spin-on” polymer coating (e.g. SU8 available fromMicrochem. Corporation)

Although a simple PLC is described with reference to FIG. 3, a personskilled in the art would recognise that the present invention is equallyapplicable to complex PLCs. For example, multiple optical componentscould be mounted on the PLC and linked via hollow core waveguide formedfrom the PLC substrate. Such PLCs could form the basis of optical signalprocessing, and/or optical signal routing and analysis system. Someexamples of such PLCs are given below.

Referring to FIG. 4 a, hollow core waveguide structures 60 a, 60 b and60 c formed in a silicon substrate 62 are shown in plan view. Angledsurfaces (e.g. surface 64) are provided to guide the light through 90°.

To minimise phase perturbations on reflection, the angled surfaces 64ideally require a surface finish that is flat to better than λ/10 ormore preferably to better than λ/20. If using a wavelength of 1.5 μm, asurface finish flat to better than 150 nm is thus required. This levelof accuracy is readily attainable using micro-fabrication techniqueswhich can typically provide a resolution of 30-50 nm.

The angled surfaces 64 thus provide mirrors that allow sections ofhollow waveguide to be orientated at any angle to one another. It wouldnot be possible to bend an optical fibre cable through such an acuteangle. If a similar circuit were to be fabricated using known SiOBtechniques, it would be necessary to provide two sections of opticalfibre with a separate (well aligned) mirror to couple light between theoptical fibre sections. The present invention can thus provide morecomplex and compact circuit layouts than prior art SiOB devices.

Although monolithic mirrors are shown in FIG. 4 a, it should berecognised that a hybrid arrangement could also provide the same opticalfunction. For example, alignment slots could be fabricated to receive inalignment polished mirrors. The hybid arrangement is useful as it allowsthe use of high optical quality mirrors that can be designed to have aminimal polarisation dependence; for example they may carry apolarisation independent multiple layer coating.

The waveguide structures described with reference to FIG. 4 a are allsubstantially straight and connected by appropriately placed mirrors.However, the hollow waveguide structures could also be curved. Forexample, and with reference to FIG. 4 b, a curved waveguide 66 formed ina silicon substrate 62 is shown. A skilled person would recognise thatthe maximum curvature attainable would depend on the guide thickness.

Referring to FIG. 5 a, a cross section through a hollow core waveguidestructure 60 of the type described with reference to FIG. 4 a is shown.The hollow core waveguide structure 60 is formed in the siliconsubstrate 62, and a silicon lid portion 68 is also provided that can beattached to the substrate 62 in the manner described above to providethe required hollow core waveguide.

As shown in FIG. 5 b, the internal surfaces of each of the walls formingthe hollow core 69 may additionally be coated with a layer of material70, for example copper, gold or silver to enhance the reflectivity ofthe 1.55 μm radiation via TIR.

If guiding linearly polarised light of known polarisation, hollow corewaveguides in which different internal surfaces have different opticalproperties can be provided to further decrease the optical lossesassociated with the waveguide.

FIG. 6 shows the Fresnel Reflection coefficient for light incident on asurface from air at an angle of 86° as a function of the refractiveindex (n) and absorption (k) of that surface for s-polarised (R_(s)) andp-polarised (R_(p)) light. It can be seen from FIG. 6 that the Fresnelreflection coefficient is strongly dependent on the polarisation of thelight. Therefore, if the polarisation state of the light that is to beguided by a rectangular hollow core waveguide is known a pair of opposedsurfaces forming the waveguide could be configured to have a lowrefractive index to optimise reflectivity for p-polarised light whilstthe second pair of opposed surfaces could be arranged to have a muchhigher refractive index to maximise reflectivity for s-polarised light.

A number of techniques are described with reference to FIG. 7 that canbe used to form waveguides in which different internal surfaces havedifferent optical properties.

FIG. 7 a illustrates a hollow waveguide formed in an SOI wafer 80fabricated using silicon on insulator (SOI) fabrication techniques. Thewafer 80 comprises an insulating layer 82 of SiO₂ material carried on asubstrate 84 and having a layer of silicon 86 located thereon. The layerof silicon 86 is etched down to the insulating layer 82 to form therequired channel 88. A lid portion 90 formed from SiO₂ material is alsoprovided.

A hollow core waveguide is thus formed having a first surface 92 and asecond surface 94 that consist of silicon, whilst a third surface 96 anda fourth surface 98 are silicon dioxide. The refractive index of siliconis around 3.5, whilst silicon dioxide has a refractive index of around1.5. Hence, optical losses in the waveguide are reduced when lightpropagating in the y-direction along the waveguide is polarised in thez-direction; i.e. there is Rs reflection at the first surface 92 andsecond surface 94 and Rp reflection from the third surface 96 and fourthsurface 98.

Referring to FIG. 7 b, a hollow waveguide 100 formed in a siliconsubstrate 102 and having a lid portion 103 is shown. The upper wall 104(i.e. the wall defined by the lid portion 103) and the lower wall 106are coated with a first material, whilst the side-wall 108 and side-wall110 are coated with a second material. The first material and secondmaterial are selected to have low and high refractive indicesrespectively in order to minimise optical losses of light polarised inthe z-direction that propagates along the waveguide in the y-direction.

Although FIG. 7 b shows coatings applied to all four walls of thewaveguide, it would be appreciated that only a single wall, or a pair ofopposed walls, could be coated as required. In other words, one or moreof the walls could remain uncoated and thus have the optical propertiesof the semiconductor material used to form the substrate.

Furthermore, physical structures can be formed in silicon to enhancereflectivity for a given polarisation of light as required. FIG. 7 cshows how a hollow core waveguide can be formed in a silicon substratethat comprises etalon side-wall structures 122. In this case, the etalonside-wall structures will enhance reflectivity. Although an etalonstructure is shown in which the hollow portions are filled with air,another material (e.g. a liquid or gas) could be used instead of air toenhance reflectivity.

The optical losses associated with hollow core waveguides can also bereduced further by controlling the shape of the waveguide core. Forexample, the wider the waveguide core, the lower the associated opticallosses. FIG. 7 d shows a rectangular cross-section waveguide 132 formedin a silicon layer 130 and having a silicon lid portion 134. The hollowcore of the waveguide 132 has a width “a” that is less than its depth“b”. Light polarised in the z-direction and propagating along thewaveguide 132 will thus experience lower losses than if it were topropagate through a waveguide of depth “a”.

It should further be noted that structures can be formed in a PLC inwhich light is guided in only one plane; for example, it could bearranged for there to be free-space propagation along a vertical axisbut waveguiding on the horizontal axis. In this case the waveguides arereferred to as planar waveguides; i.e. they only guide in one plane.Planar waveguides may be employed where beam expansion in one dimensionis required whilst constraining the beam width by guiding in a seconddimension. If guiding is required in only the horizontal plane, a lidportion is not required. The converse situation is also possible wherelight is guided between the lid and the floor of the waveguide but notin the lateral plane.

Referring to FIG. 8, it is demonstrated how beam splitters andpolarisation filters may be formed in a hollow waveguide PLC.

FIG. 8 a shows a beam splitter fabricated from hollow waveguidestructures formed in a silicon substrate 160. The beam splittercomprises an input hollow core waveguide 162, a first output hollow corewaveguide 164 and a second output hollow core waveguide 166. Lightpropagating through the input hollow core waveguide 162 is partiallyreflected from a thin silicon wall 168 into the first output hollow corewaveguide 164 and also partially transmitted and coupled into the secondoutput hollow core waveguide 166.

The angle (θ) between the input hollow core waveguide 162 and the firstoutput hollow core waveguide 164 determines the angle of incidence oflight on the thin silicon wall 168. As shown in FIG. 8 c, thereflectance properties of silicon depends on both the angle ofincidence, and the polarisation state, of the incident light. Therelative proportion of the power coupled from the input hollow corewaveguide 162 into the first and second output hollow core waveguides164 and 166 can thus be selected by fabricating the device with acertain angle (θ).

Furthermore, as shown in FIG. 8 b, a polarisation splitter can befabricated by arranging for the angle θ to equal the Brewster angle. Inthis case, an angle of θ=74° will result in light polarised in thez-direction being routed from the device via the first output hollowcore waveguide 164 whilst light polarised in the x-direction will berouted from the device via the second output hollow core waveguide 166.

An etalon filter may be formed in a device of the type described withreference to FIGS. 8 a and 8 b instead of the thin silicon wall 168.This would provide an optical element that would have differentreflective properties for different wavelengths of light, and hence thedevice could also operate as a spectral filter.

Although a monolithic beam splitter and Brewster plate are described,the skilled person would also appreciate that a similar optical functioncould be implemented using a hybrid arrangements. Alignment slots couldbe formed in the substrate to receive the necessary optical components.

Referring to FIG. 9, it can be seen how the silicon material of asubstrate can also be formed to provide a light focussing function.

FIG. 9 a shows a silicon substrate 190 in which a silicon lens structure192 and hollow core waveguide 194 have been formed. The lens structure192 will act as a lens to enable light 196 guided along the hollow corewaveguide 194 to be focussed to a point 198. Such lenses may be used,for example, to focus light to a detector element.

As shown in FIG. 9 b, a shaped silicon reflector 200 can also be formedin a silicon substrate 202 to optically link hollow core waveguides. Thereflector 200 performs the function of routing light through a certainangle (in this case 90°) from a first hollow core waveguide 204 to asecond hollow core waveguide 206 whilst also focussing light 208. Again,such an element may be used in a variety of different ways in PLCs andwould be relatively simple to realise as it does not require ananti-reflection coating.

Although a PLC of the type described herein may comprise a completeoptical circuit, it may also be necessary to couple light into or out ofa PLC, typically via optical fibres.

FIG. 10 shows a PLC formed in a silicon substrate 220 and arranged toreceive light from a first input optical fibre 222. The input opticalfibre 222 has a hollow core, and light therefrom is coupled into theinput hollow core waveguide 224 using an input ball lens 226. Lightpropagating along the input hollow core waveguide 224 is directed to anetalon structure 228. The etalon structure 228 spectrally filters lightinto the first output hollow core waveguide 230 or the second outputhollow core waveguide 232 depending on its spectral characteristics.Light propagating through the first output hollow core waveguide 230 iscoupled in to a first output optical fibre 234 via ball lens 236, andlight propagating through the second output hollow core waveguide 232 iscoupled in to a second output optical fibre 238 via ball lens 240.Again, a hybrid etalon filter could be used instead of the monolithicelement shown.

Although ball lenses are shown in FIG. 10, other lenses such as GRIN rodlenses may alternatively be used. The etalon structure 228 may also bereplaced with beam splitter or a Brewster plate as necessary. Theoptical fibre may be single or multiple mode as required.

SOI technology is particularly suited to forming PLCs to which opticalfibre are coupled. This is because typical SOI wafers comprise a siliconlayer that has a thickness which is very accurately defined during themanufacturing process. In the fabrication of hollow waveguide structuresin the silicon layer of a SOI wafer, the silica insulating layer acts asa vertical “stop” as far as the etching process is concerned. SOItechniques can thus provide sub-μm channel depth accuracy.

The SOI etching accuracy should be contrasted to channel etching in puresilicon which has an accuracy around a few percent of the etch depth.Etching a channel in a pure silicon wafer to take a fibre (stripped toits cladding diameter of 125.0 μm) would produce a 3.0 μm to 4.0 μminaccuracy in the depth of the etch channel. As the core of the fibre istypically only 10.0 μm in diameter a vertical misalignment of thismagnitude when coupling to/from a fibre from/to some other component(e.g. from a semiconductor laser) could prove detrimental. Therefore, anSOI based fabrication route would have advantages for alignment andwaveguide cross-section accuracy which would reduce polarisationdependent losses.

As described above, a PLC of the present invention could comprise hollowcore waveguides that allow single or multiple mode propagation. Incertain circumstances it may also be necessary to alter the dimensionsof the hollow core waveguide; e.g. to efficiently couple light into orout of different optical components.

Referring to FIG. 11 a, a hollow core waveguide structure 260 formed ina silicon substrate 262 is shown. A wide (125 μm) diameter outputwaveguide 264 is optically linked to a narrower (62.5 μm) diameter inputwaveguide 266 via a tapered waveguide portion 268. The length of thetapered portion is 1.875 mm.

FIG. 11 b shows the intensity field of light propagating in the outputwaveguide 264 that results from a fundamental mode input beam in theinput waveguide 266. As shown in FIG. 11 c, the output light in theoutput waveguide 264 is propagating predominantly in the fundamentalmode. In other words, the tapered waveguide allows expansion of the beamsize whilst ensuring the majority of the output beam power is coupledinto the fundamental mode.

PLCs of the present invention may also comprise hollow core multi-modeinterference (MMI) devices formed in the substrate. An example of a beamsplitting and beam recombining MMI device is given in U.S. Pat. No.5,410,625. Variations and improvements to the basic MMI devices of U.S.Pat. No. 5,410,625 are also known. For example, U.S. Pat. No. 5,379,354describes how variation of input guide location can be used to obtain amulti-way beam splitter that provides division of the input radiationinto outputs beams having differing intensities. Use of MMI devices toform laser cavities has also been demonstrated in U.S. Pat. No.5,675,603. Various combinations of MMI splitter and recombiner deviceshave also been used to provide an optical routing capability; forexample, see U.S. Pat. No. 5,428,698. In all the above cases, the MMIdevice could be fabricated as hollow core waveguides in silicon, or anyother appropriate semi-conducting material, and form an integral part ofthe PLC.

The MMI device may be fabricated from a multi-mode region formed in thesubstrate to which input and output single mode optical fibre cables arecoupled. In this manner, beam splitting/combining can be obtained inwhich the split beams are images of the input beam.

In particular, rectangular or square cross-section hollow multi-modewaveguides can be designed to provide re-imaging of symmetric,anti-symmetric or asymmetric optical fields by designing the length ofthe waveguide to have an appropriate relationship to its width. Forexample, for a symmetric field in a square sectioned waveguide there-imaging length is given by the square of the waveguide width over thewavelength of the propagating radiation, i.e. L=w²/λ, where L, is theguide length, w, is its width, and, λ is the wavelength of theradiation. Re-imaging of the symmetric field occurs at this length andmultiples of this length, i.e. at n.w²/λ, where, n, is an integernumber.

For the case of a 50.0 μm wide hollow waveguide and 1.55 μm radiation,the re-imaging length is given by 50²/1.55=1613 μm=1.613 mm. Thesymmetric field would be re-imaged at this length and also at integermultiples of this length, i.e. 3.23 mm, 4.84 mm etc. For example, aTEM₀₀ gaussian input beam from a single mode optical fibre could bere-imaged at distances of 1.613 mm. At the re-imaging points anyrequired optical components could be situated. In this manner there-imaging phenomena provides an additional way of guiding light betweena series of components.

Alternatively, for the case of an asymmetric optical field, re-imagingoccurs at eight times the length required for symmetric fieldre-imaging, i.e. at 12.09 mm (8×1.613 mm) for a 50.0 μm wide hollowwaveguide. A mirror image of the asymmetric field is also formed at halfthis length i.e. at 6.05 mm.

In the case of a rectangular waveguide where the horizontal and verticalwidths of the waveguide are substantially different the re-imaginglengths associated with the two widths are themselves different.However, by arranging that the relationship between the widths of therectangular hollow waveguide is such that re-imaging is produced atidentical lengths for each width, any field can be re-imaged.

For example, a symmetric field can be re-imaged in a hollow rectangularwaveguide by arranging that the re-imaging lengths, L₁=n₁.w₁ ²/λ, and,L₂=n₂.w₂ ²/λ, associated with axes of width w₁ and w₂, are identical.This can be achieved by making w₂=(n₁/n₂)^(1/2).w₁, here, as previously,n₁ and n₂ are integer numbers.

Another type of MMI device suitable for inclusion in a PLC of thepresent invention is the wavelength de-multiplexer described inco-pending PCT patent application number GB2002/004560 and shown in FIG.12.

The demultiplexer 300 is formed in a silicon SOI substrate 302 andcomprises an input fundamental mode waveguide 304, a central multi-moderegion 306 and four output waveguides 308 a-308 d (collectively referredto as 308). The dimensions and positions of the waveguides are selected(as described in GB2002/004560) such that the four wavelengthscomponents entering the multi-mode region 306 from the input fundamentalmode waveguide 304 are separated and separately output via the outputwaveguides 308.

It is also possible to form PLC devices in which light is proximitycoupled into adjacent waveguide. Referring to FIG. 13, a first hollowcore waveguide 340, a second hollow core waveguide 342 and a thirdhollow core waveguide 344 formed in a silicon substrate 346 and having alid portion 347 are shown. The thickness “c” of the silicon walls 348and 350 is sufficiently thin to enable light to be transmitted toadjacent waveguides. A proximity coupler component of this type may beused as beam splitter; for example to tap off a small percentage of apropagating beam without having to insert a beam-splitting componentinto the optical path.

Referring to FIG. 14, the PLC may comprise both hollow and solid core(e.g. “ridge”) silicon waveguides. This enables the realisation ofoptical functions in both solid and hollow core technologies.

FIG. 14 a shows a Brewster interface between a hollow core waveguide 400and a solid core waveguide 402 both of which are formed on an SOIsubstrate. FIG. 14 b shows a cross-section along B-B of the hollow corewaveguide 400, and FIG. 14 c shows a cross-section along A-A of thesolid core waveguide 402. The hollow core waveguide 400 terminates atthe angled interface 404 of the solid core waveguide 402. The waveguide400 and 402 are arranged such that the interface is at the Brewsterangle. This provide efficient coupling between the hollow and solid corewaveguides.

Referring to FIG. 15, a hollow core waveguide 450 and an angled surface452 are shown. The surface 452 is angled at approximately 45° to theplane of the substrate such that light 454 is coupled out of the planeof the substrate. The arrangement shown in FIG. 15 may be used to couplelight into or from other circuits or devices located in a differentvertical plane to the plane of the substrate. In this manner a threedimensional stacked PLC (e.g. a three dimensional optical switch) couldbe produced.

One way of monolithically fabricating such a mirror is a precisionoff-axis cut in [100] silicon material that is offset at an angle ofabout 8-9°. Numerous alternative ways to manufacture such an angledsurface would be apparent to a person skilled in the art. Hybrid mirrorarrangements could also be used.

Referring to FIG. 16, experimental data showing the guide lengthdependent optical transmission characteristics of hollow core opticalwaveguides suitable for incorporation in a PLC of the present inventionare shown.

Curve 500 shows the predicted, and points 502 a-502 c the measured,optical transmission of a hollow core waveguide formed in a siliconsubstrate having a square internal core of 50 μm width and depth. Curve504 shows the predicted, and points 506 a-506 c the measured, opticalproperties of a similar waveguide in which a copper coating has beenapplied to each of its internal surfaces.

Curve 508 shows the predicted, and points 510 a-510 c the measured,optical transmission of a hollow core waveguide formed in a siliconsubstrate having a square internal core of 125 μm width and depth. Curve512 shows the predicted optical properties of a similar waveguide inwhich a copper coating has been applied to each of its internalsurfaces. In all cases shown in FIG. 16 radiation having a wavelength of1.55 μm was used.

It can thus be seen that increasing the dimensions of a waveguidereduces optical losses and the inclusion of a reflective coating (inthis case copper) reduces losses even further. However, allowableangular alignment tolerances are reduced.

Referring to FIG. 17, the effect of angular alignment of components isshown.

FIG. 17 a shows a silicon substrate 600 in which a first hollowwaveguide 602, second hollow waveguide 604 and a third hollow waveguide606 are formed. A beam splitting element 608 is located in alignmentslot 610. It can be seen that the element 608 has an angularmisalignment (∂θ) determined by the element and slot manufacturingtolerances.

FIG. 17 b shows the power coupling efficiency into the various modes ofa hollow core waveguide as a function of angular misalignment (∂θ).Curve 620 shows the power coupled into the fundamental mode, whilstcurves 622 show the optical power coupled into the higher order modes.

Referring to FIG. 18, the effect of lateral alignment is demonstrated.

FIG. 18 a shows a first hollow core wave guide 650 laterally displacedfrom a second hollow core waveguide 652 by ∂l. The power couplingcoefficient as a function of lateral displacement is shown in FIG. 18 bwhere curve 654 shows the power coupled into the fundamental mode,whilst curves 656 show the optical power coupled into higher ordermodes.

It can be seen from the above that fundamental mode propagation throughan integrated system of components interconnected by hollow waveguidescan be attained if the waveguide dimensions and alignment tolerances areappropriately selected. This is especially important in a system ofcomponents which couples light to/from single mode optical fibresbecause the amount of power in the fundamental waveguide mode dictateshow much light is coupled to/from the single mode fibre. Ensuring highefficiency fundamental mode propagation in the waveguides ensures goodcoupling to the fundamental mode of a single mode fibre and an overalllow insertion loss.

In other words, there is a trade off between the width of the waveguideand the angular and lateral alignment tolerances that are required (ofboth waveguides and components) in order to ensure that efficientfundamental mode propagation is achieved. Lower attenuation coefficientscan be obtained by making the guide cross-section (width) large enoughbecause the attenuation coefficient is inversely related to waveguidewidth. Making the waveguide width larger also eases lateral alignmenttolerances, but it can be seen to tighten angular alignment tolerances.

Referring to FIG. 19, a technique for ensuring accurate alignment ofcomponents placed in a slot is shown.

A silicon substrate 700 has a slot formed therein to hold an opticalcomponent 702. A number of spring clips 704 (also termed micro-grippers)are formed in the silicon by known micro-fabrication techniques. Theseclips 704 are such that when displaced they provide a lateral force. Inthis manner, the component is held firmly in alignment in the slot.

Although FIG. 19 shows clips surrounding the optical component, it isalso possible to press the component against a reference surface such asthe side-wall of the slot. It would also be appreciated by the skilledperson that springs or other MEMS features fabricated by the removal ofa sacrificial layer of oxide in a silicon wafer would result in acertain amount of undercut. This undercut would have no effect ifassociated with alignment slots, and would also make little differenceto propagation in a rectangular hollow waveguide where the modedistribution is typically circular or elliptical.

A PLC of the present invention could be used to implement numerousdifferent optical circuits. A few examples of these includeinterferometers (e.g. Michelson or Mach-Zender), spectrometers, lidarand optical readout of MEM devices (such as sensors or actuators).Telecoms circuits (routers, multiplexers, demultiplexers etc) could alsobe implemented. Although optical components are described above, thereis no reason why the PLC could not alternatively or additionallycomprise microwave components and hollow core waveguides to guide themicrowave radiation. Opto-microwave integration in a single circuitwould thus be possible.

1. A photonic light circuit device comprising a semiconductor substrateand a plurality of optical components, wherein one or more hollow coreoptical waveguides are formed in the plane of the semiconductorsubstrate to optically link said plurality of optical components,characterised in that each of the plurality of optical components isretained in an alignment slot formed in the semiconductor substrate,each alignment slot being arranged to define the alignment of theoptical component retained therein with respect to the one or morehollow core optical waveguides, the alignment slot being separate fromthe hollow core optic waveguides.
 2. A device according to claim 1 andfurther comprising at least one optical component formed from thematerial of the semiconductor substrate.
 3. A device as claimed in claim1 wherein the semiconductor substrate comprises silicon.
 4. A device asclaimed in claim 1 wherein the semiconductor substrate comprises asilicon on insulator (SOI) wafer.
 5. A device as claimed in claim 1wherein the semiconductor substrate forms a base portion of the photoniclight circuit device and a lid portion is additionally provided to formsaid one or more hollow core optical waveguides.
 6. A device as claimedin claim 5 wherein one or more optical components are attached to thelid portion.
 7. A device as claimed in claim 5 wherein the lid portioncomprises semiconductor material.
 8. A device as claimed in claim 7wherein the semiconductor material at the lid portion is silicon.
 9. Adevice as claimed in claim 7 wherein one or more optical components areformed in the semiconductor material of the lid portion.
 10. A deviceaccording to claim 1 wherein at least some of the internal surfaces ofsaid one or more hollow core optical waveguides carry a reflectivecoating.
 11. A device as claimed in claim 10 wherein the reflectivecoating comprises one or more layers of material to provide a surfacehaving an effective refractive index lower than that of the waveguidecore within the operating wavelength band.
 12. A device as churned inclaim 11 wherein the reflective coating comprises at least one layer ofany one of gold, silver or copper.
 13. A device as claimed in claim 11wherein the reflective coating composes at least one layer of dielectricmaterial.
 14. A device as claimed in claim 11 wherein the reflectivecoating comprises at least one layer of Silicon Carbide.
 15. A device asclaimed in claim 1 wherein at least one c the one or more hollow coreoptical waveguides support fundamental mode propagation.
 16. A device asclaimed in claim 1 wherein at last one of the one or more hollow coreoptical waveguides support multi-mode propagation.
 17. A deviceaccording to claim 16 wherein the multi-mode region is of a length suchthat re-imaging occurs.
 18. A device as claimed in claim 1 wherein atleast one of the one or more hollow core optical waveguides has asubstantially rectangular internal cross-section.
 19. A device asclaimed in claim 18 wherein at least one of the one or more hollow coreoptical waveguides has a substantially square internal cross-section.20. A device as claimed in 18 in which the rectangular hollow coreoptical waveguide has a first cross-sectional dimension parallel to afirst waveguide wall and a second cross-sectional dimension orthogonalto said first cross-sectional dimension wherein the first cross-sectiondimension is at least 10% greater than the second cross sectionaldimension.
 21. A device as claimed in claim 18 wherein the refractiveindices of the surfaces defining the at least one rectangular internalcross-section hollow core optical waveguide are substantially equal. 22.A device as claimed in claim 18 wherein opposite surfaces forming therectangular internal cross-section hollow core optical waveguide havesubstantially equal effective refractive indices and adjacent surfacesforming the rectangular internal cross-section hollow core opticalwaveguide have different effective refractive indices.
 23. A device asclaimed in claim 22 wherein a pair of opposed surfaces of therectangular internal cross-section hollow core optical waveguide carry ahigh refractive index coating.
 24. A device as chimed in claim 1 foroperation with radiation within the wavelength range of 0.1 μm to 20 μm.25. A device as claimed in claim 1 for operation with radiation withinthe wavelength bands of 3 μm to 5 μm.
 26. A device as claimed in claim 1for operation with radiation within the wavelength bands of 8 μm to 12μm.
 27. A device as claimed in claim 1 for operation within thewavelength bands of 1.4 μm to 1.6 μm.
 28. A device according to claim 1wherein the semiconductor substrate comprises at least one alignment lotarranged to receive an optical fibre cable and to optically couple saidoptical fibre cable with one of said one or more hollow core opticalwaveguide of the semiconductor substrate.
 29. A device according toclaim 28 wherein a mode matching means is additionally provided in thevicinity of the alignment slot to allow coupling between a the modes ofan optical fibre and the analogous modes of a hollow core optical wayguide of a different core diameter.
 30. A device according to claim 29wherein the mode matching means is any one of a GRIN or ball lens.
 31. Adevice according to claim 28 wherein the alignment slot is arranged toreceive a hollow core optical fibre.
 32. A device according to claim 28wherein the alignment slot is arranged to received a tensed opticalfibre.
 33. A device according to claim 1 wherein at least one of saidtwo or more optical components comprises a micro-electro-mechanical(MEMs) device.
 34. A device according to claim 1 wherein at least one ofsaid two or more optical components comprises a mirrored surface that isangled to direct light out of the place of the semiconductor substrate.35. A device as claimed in claim 1 and further comprising at least onemicro wave component.
 36. A device as claimed in claim 1 wherein thesemiconductor substrate additionally comprises a hollow core microwavewaveguide.
 37. A base portion for a photonic light circuit deviceaccording to claim 1 comprising a semiconductor substrate comprising aplurality of alignment slots for retaining optical components and one ormore hollow channels formed therein, wherein said base portion isarranged such that when combined with an appropriate lid portion at leon hollow core optical waveguide is formed.
 38. A base portion for aphotonic light circuit of claim 1 comprising a semiconductor substratein which one or more hollow waveguide channels and a plurality ofalignment slots to receive and align an optical component.
 39. A methodof fabricating a photonic light circuit comprising the steps of taking abase portion as claimed in claim 37 and attaching a lid thereto.
 40. Amethod of fabricating a photonic light circuit device comprising thestops of micro-fabricating one or more hollow channels and a pluralityof alignment slots in a semiconductor substrate wherein the one or morehollow channels act, in use, as hollow core optical waveguides and eachof the plurality of alignment slots is separate from the hollow channelsand arranged to passively align an optical component retained therein.41. A method as claimed in claim 39 and comprising the ad step ofcoating the internal surfaces of the hollow channel(s) with a layer ofmaterial having a refractive index lower than that of the waveguide corewithin the operating wavelength band.
 42. A method of forming a photoniclight circuit comprising the steps of; (a) taking a semiconductorsubstrate in which at least one hollow core optical waveguide andplurality of slots to receive an optical component are formed using themethod of claim 40, and (b) introducing an optical component into the atleast one slot, whereby the step of introducing the optical componentinto the at least one slot also acts o as to align said opticalcomponent.